The long-range goal of this laboratory is to understand how signals initiated from the TCR and CD28 are integrated and transduced to downstream transcription factors. This knowledge is important for rational development of strategies and therapies to modulate immune responses in the treatment of cancer and various immunological disorders such as autoimmune diseases. The objective of this application is to elucidate the signaling function of a recently cloned MAP3K, NIK, in transducing the T cell co-stimulatory signals. Preliminary studies demonstrate, by both genetic and biochemical approaches, that NIK plays a critical role in T cell co-stimulation. It was shown that NIK is essential for TCR/CD28- mediated activation of different transcription factors, including NFkB and AP-1, both serving as key regulators of a CD28-responsive IL-2 gene enhancer, CD28RE/AP-1. This finding indicates that NIK may serve as a key signaling component transducing the TCR/CD28 signals to different downstream signaling pathways. As seen with many signaling molecules, NIK contains a number of structural motifs known to mediate protein- protein interactions. The central hypothesis to be tested is that NIK interacts with multiple signaling molecules in the T cell co-stimulatory pathway, mediating activation of different downstream kinases and transcription factors. There are three specific aims: (1) Investigate the signaling mechanisms through which NIK regulates downstream transcription factors; (2) Define the structural basis for the signaling function of NIK; and (3) Clone and characterize novel NIK-interacting proteins. At the completion of the project, it is expected that it will be known how NIK functions in signal transduction pathways connecting the TCR/CD28 signals to activation of downstream CD28RE/AP-1 regulatory factors. It is also expected that novel signaling molecules will have been isolated which function in the T cell co-stimulatory pathway.